The subject matter disclosed herein relates to magnetic resonance imaging systems and, more particularly, to transversely folded gradient coils for producing substantially linear transverse magnetic fields in such systems.
Magnetic resonance imaging systems may include one or more magnetic gradient field coils configured to produce predefined magnetic fields when energized. It is known in the present state of the art to provide an actively-shielded transverse coil apparatus 10, such as shown in the simplified schematic view of FIG. 1. The coil apparatus 10 includes an outer coil section 11 and an inner coil section 21 disposed within the outer coil section 11. Both the outer coil section 11 and the inner coil section 21 are substantially cylindrical surfaces. The outer coil section 11 and the inner coil section 21 are shown concentrically positioned on a longitudinal axis 19, commonly referred to as the gradient axis, and are usually specified in the relevant art as the z-axis of a Cartesian coordinate system. The coil apparatus 10 typically includes a number of gradient coils for producing the desired, primary magnetic field in an imaging volume 20 when powered by electrical current.
In the example shown, the inner coil section 21 includes a first inner gradient coil 23, here shown disposed to the left of the x-y plane, and a second inner gradient coil 25, here shown disposed to the right of the x-y plane. Each inner gradient coil 23 and 25 extends substantially across half the circumference of the inner coil section 21, both here shown lying above the y-z plane. In a typical configuration, the inner coil section 21 includes third and fourth inner gradient coils (not shown for clarity of illustration) disposed on the lower surface of the inner core section 21 (i.e., below the y-z plane). Each of the four inner gradient coils includes a folded-loop current path (not shown for clarity of illustration) configured such that, when powered, each folded-loop current path generates a respective magnetic gradient field component in the imaging volume 20.
The outer coil section 11 includes a first outer gradient coil 13 disposed radially outward of the first inner gradient coil 23, and a second outer gradient coil 15 disposed radially outward of the second inner gradient coil 25. Each outer gradient coil 23 and 25 extends substantially across half the circumference of the outer coil section 11 so as to cancel or shield that portion of the primary magnetic field generated by the inner gradient coils 23 and 25 that might otherwise extend radially beyond the outer coil section 11. This shielding is accomplished by means of an opposing magnetic field component generated by a folded-loop current path (not shown for clarity of illustration) provided on each of the outer gradient coils 23 and 25. The current in each outer folded-loop current path flows opposite to the current flow in the adjacent inner folded-loop current path to generate the opposing magnetic field component. The outer coil section 11 also includes third and fourth outer gradient coils (not shown) on the outer coil section 11 lying below the y-z plane and disposed radially outward of the corresponding third and fourth inner gradient coils 23 and 25. A current return path (not shown) is provided for transport or continuity of the current flowing in the outer gradient coil 13 and the inner gradient coil 23 to produce a closed coil pattern on the surfaces of the coil sections.
U.S. Pat. No. 5,349,381 “Double type coil for generating slant magnetic field for MRI”, for example, discloses a gradient coil comprising spiral-shaped current path patterns formed on double semi-cylindrical surfaces having a common axis. The current paths are connected in series to form a single current path, the disclosed design providing a folded return current path between a primary surface and a shield surface. U.S. Pat. No. 5,512,828 “Actively shielded transverse gradient coil for nuclear magnetic resonance tomography apparatus” discloses an actively shielded transverse gradient coil arrangement having a longitudinal folding of current paths between a primary coil and a secondary coil.
However, although current folding may increase coil efficiency and improve shielding, manufacturing complexity may be increased. Moreover, the folded configuration limits physical access from the end of the gradient coil to the space between the primary surface and the shield surface. One configuration intended to improve these shortcomings is disclosed in U.S. Pat. No. 5,886,548 “Crescent gradient coils,” in which current conductors are wrapped in a crescent shaped arrangement. The crescent-shaped, axially aligned coils may also be used in conjunction with Golay-type coils. Such transversely wound coils, however, may fail to meet certain gradient linearity, uniformity, or leakage field requirements specified for imaging applications.
What is needed is a distributed-current transverse gradient coil design that produces a linear transverse gradient magnetic field and overcomes the shortcomings of the prior art.